1. Field of the Invention
This invention relates to a novel electrolyte used in the field of electronics, such as cell devices, photochromic devices and sensor devices, and a secondary cell (storage cell) making use of such an electrolyte.
2. Related Background Art
In 1973, Wright et al. reported ion conductivity of a complex of polyethylene oxide (PEO) with an alkali metal salt and, in 1979, Armand et al. demonstrated a possibility of an electrolyte used in cells. This has brought about a world-wide spread of researches on solid electrolytes. Solid electrolytes are not liquid in form, and hence, as being free from any leakage to the outside, they are advantageous over liquid electrolytes in respect of heat resistance, reliability, safety, and device miniaturization. Also, organic matter is softer than inorganic matter, and hence has an advantage that it can be worked with ease. In general, the ion conductivity of electrolytes is expressed as the product of carrier density and electric-charge or ion mobility, and hence electrolytes are required to have a high polarity for dissociating ions and a low viscosity for migrating ions dissociated. From this point of view, PEO can not be said to have sufficient properties as a solid electrolyte. First of all, the ion transport mechanism of PEO is based on the ligand exchange in which ions dissociated as a result of coordination to donative polar-group moieties are delivered one after another on account of segment movement caused by heat. Hence, the PEO tends to have temperature dependence. Also, dissolution of metal ions in a large quantity for the purpose of enhancing carrier density may cause crystallization to lower ionic mobility conversely. In order to prevent this crystallization, development has been made on a PEO formed by urethane cross-linking (M. Watanabe et al., Solid State Ionics, 28-30, 911, 1988) and also on a PEO to the cross-linked moieties of which side chains have been introduced in order to improve ionic mobility (The 40th Polymer Forum Draft Collections, 3766, 1991). Recently, development has also been made on a molten salt type PEO to a terminal of which a salt has been introduced (K. Ito et al., Solid-State Ionics, 86-88, 325, 1996, and K. Ito et al., Electrochim. Acta, 42, 1561, 1997). However, since any sufficient ion conductivity has not yet been attained under existing circumstances, prevalent are electrolytic solutions formed of a mixture of a solvent having a high dielectric constant and a solvent having a low viscosity, or gel electrolytes formed of an electrolytic solution immobilized with an organic polymer. Also, when solid electrolytes are used as cell devices, not only the efficiency of ion transport but also the efficiency of electrochemical reaction at surfaces in contact with electrodes have come into question.
Meanwhile, discotic liquid-crystal phases are liquid-crystal phases discovered by S. Chandrasekhar et al. in 1977 (Pramana 9, 471, 1977). For example, as explained by the same authors in Rep. Prog. Phs. 53 (1990) 57, entitled xe2x80x9cDiscotic Liquid Crystalsxe2x80x9d, and by Shunsuke Takenaka in Quarterly Chemistry Outlines Vol. 22, page 60, The Chemical Society of Japan, entitled xe2x80x9cDesign And Synthesis of Discotic Liquid Crystalsxe2x80x9d, such phases are seen in compounds in which a plurality of relatively long side chains are linked with disk-like cores. The type of such compounds can be classified chiefly by structure of the cores, and may include derivatives of hexa-substituted benzene and tri-substituted benzene, derivatives of phthalocyanine and porphyrin, derivatives of triphenylene, truxene and pyrylium, tribenzocyclonene derivatives, azacrown derivatives, and cyclohexane derivatives. On account of structural features of discotic liquid crystals, some reports are made which suggest their application to devices, as exemplified by application of electron (or hole) channels in systems having conjugated pi electrons, such as phthalocyanine and triphenylene (Piechocki et al., J. Am. Chem. Soc., 1982, 104, p.5245), and, in the case of cyclic ones whose cores are formed of azacrown, application of molecular channels in which molecules pass selectively through gaps at the centers (J. Chem. Soc., Chem. Commun., 1985, 1794, and J. Chem. Soc., Chem. Commun., 1995, 117, 9957).
Recently, Kato et al. make a report also on electrolytes having a smectic liquid-crystal phase (1998 Japan Liquid-Crystal Association Lecture Draft Collections, 3B08). Also, Japanese Patent Application Laid-Open No. 11-86629 discloses an ion-conductive material that utilizes orientation properties of liquid crystal. There, however, is no disclosure as to use of a mixture of a liquid-crystal compound, a straight-chain or branched polyether compound and a metal salt.
The present invention was made taking account of such conventional techniques. Accordingly, an object of the present invention is to provide an electrolyte having anisotropy in ion conductivity, containing a liquid-crystal compound, a straight-chain or branched polyether compound and a metal salt.
Another object of the present invention is to provide a secondary cell making use of such an electrolyte.
The present invention provides an electrolyte comprising a liquid-crystal compound, a straight-chain or branched polyether compound and a metal salt.
The liquid-crystal compound may preferably be a discotic liquid-crystal compound.
The liquid-crystal compound may also preferably be a liquid-crystal compound having at least a smectic phase.
The liquid-crystal compound may still also preferably have been made polymeric by polymerization reaction.
The metal salt may preferably be an alkali metal salt.
The electrolyte may preferably contain an organic solvent.
The present invention also provides a secondary cell comprising the electrolyte described above.
According to the present invention, an electrolyte having anisotropy in ion conductivity can be provided for electrolytes used in the field of electronics, such as cells and sensor devices.
According to the present invention, a secondary cell can be provided which make use of the above electrolyte having anisotropy in ion conductivity.